This invention relates to a surface acoustic wave (SAW) filter and, more particularly, to a surface acoustic wave filter comprising a plurality of output ports.
As well known in the art, a surface acoustic wave filter comprises a piezoelectric substrate on which an input interdigital transducer and a plurality of output interdigital transducers are disposed with the input interdigital transducer opposed to the output interdigital transducers. The input interdigital transducer is connected to an input port while the output interdigital transducers are connected to a plurality of output ports, respectively. The output interdigital transducers have weighting factors which are different from one another.
Such a surface acoustic wave filter is, for example, disclosed in an article which is contributed by SHIGEO MATSU-URA et al to IEEE TRANSACTIONS ON SONICS AND ULTRASONICS, VOL. SU-28, No. 3 (May 1981), pages 156-161, and which has a title of "TV Tuning Systems with SAW Comb Filter".
In order to obtain a desired filtered output signal, the output ports are connected to a filter output terminal through a plurality of switches, respectively. One of the switches is selectively turned on in a conventional surface acoustic wave filter. In other words, the conventional surface acoustic wave filter carries out a switching of filter characteristics by selecting one of the output interdigital tranducers. In general, as the switch is used a PIN diode or a mechanical switch.
With this structure, the number of components is increased as the number of the output ports, namely, of the output interdigital transducers is increased. This will cause problems of reduced reliability and higher costs besides the complicated circuit structure.
In addition, when the surface acoustic wave filter comprises first through n-th output ports where n represents a positive integer which is not less than two, the filtered output signal is outputted from one of the first through the n-th output ports. It will be assumed that each of the first through the n-th output ports has a port reliability Rp, namely, a port Fit number. Under the circumstances, the surface acoustic wave filter has a filter reliability Rf, namely, a filter Fit number which is equal to the port reliability Rp to the n-th power, that is: EQU Rf=Rp.sup.n.
As a result, there is another problem that the filter reliability Rf (the filter Fit number) is reduced as the number of the output ports is increased.